Modern display systems typically employ bistable or analog electronic devices. These include cathode ray tube displays, light emitting diode displays and liquid crystal displays. Two of these systems, namely, cathode ray tubes and light emitting diodes suffer from the problem of relatively high power consumption and poor visibility during high ambient light conditions (e.g. sunlight). While liquid crystals solve the problem of power consumption and loss of readability under high levels of illumination, nevertheless, their contrast is generally quite poor under all conditions, thus rendering them difficult and time consuming to interpret. Interpretation time is particularly important in many applications such as advertising where consumers' attention must be attracted and held within the duration of a glance or an instrumentation application where operation decisions must be made quickly.
In recent years, interest has been renewed in the use of electromechanical systems for display purposes. One of the earliest uses of the same was in the area of clocks where cards having half numerals painted on them are rotated on an axle to sequentially display numerals signifying the time. Bistable systems have also been proposed. In U.S. Pat. No. 3,648,281, Dahms suggests that a display panel can be made by constructing a device having a dark panel and a light panel and magnetically supporting a flag having a dark side and a light side between the panels and driving the device electrostatically to expose alternately the dark side of the flag and the dark panel or the light side of the flag and the light panel. However, while this idea was first proposed in the late 1960's, it has failed to see any practical implementation, largely because of the difficulties involved in manufacturing and maintaining such display structures. Still another problem with Dahms' device was the fact that the flag had to traverse a relatively wide angle (the wide angle is required in order to have a wide angle of view with the device), resulting in excessively long time periods for changing the state of the display and relatively high driving voltages.
One bistable system which has seen relatively wide employment is a panel that incorporates a matrix of hundreds of rotatable coin-shaped disks with black painted front faces and bright reverse sides mounted on a black background panel. Each of these disks are rotated or "flipped" by one of hundreds of electromagnets, one of which is connected to each of them to expose the bright color on their reverse. However, despite its employability in such areas as bus destination signs, this sort of system is very expensive to manufacture, requires expensive drive circuitry and can only be operated slowly.
U.S. Pat. No. 3,772,537 of Clifford et al. suggested that a metal coated plastic film coiled at one end of a device could be electrostatically unfurled to block a light source. While, this structure is essentially unsuited for use in a display, it is noted that, to the best knowledge of the applicant, such systems have not seen any commercial employment, probably due to the inappropriateness of this structure for large area multi-element displays.
In the early 1970's, I conceived a device shown in U.S. Pat. No. 3,897,997 comprising a pair of fixed electrodes and a flexible tongue disposed between them. Whereas this configuration would have been regarded as impractical because the electrodes which form the panels are not nearly in the plane of display of the device, I solved this problem by making the flexible tongue electrode reflective. Because the tongue is flexible and because of the configuration of the fixed electrodes, I was also able to replace the magnetic hinge of Dahms with a simple flexible mount.
In my later U.S. Pat. No. 4,094,590, I disclosed a tongue mounting structure which greatly reduces criticality of the manufacturing techniques used to manufacture my electrostatic display device through the use of a wrinkle reducing stressed tongue. In my co-pending U.S. patent application Ser. No. 103,995, entitled REFLECTIVE VIDEO DISPLAY AND METHOD OF MAKING SAME, now U.S. Pat. No. 4,336,536, I have refined the optics of the reflective electrostatic display device by the use of a pair of flat reflective configurations for the tongue, which configurations are at a relatively small angle with respect to each other, whereby an extremely wide field of view is obtained.
Generally, the devices covered by my earlier patents operate by mounting the tongue in such a manner that it is at rest positioned adjacent one electrode in one of the display states and driving it to the other display state by electrostatically attracting it to the other fixed electrode. While this arrangement has been found to work well, under certain conditions operation becomes erratic, apparently due to relatively permanent changes in the electrostatic characteristics of the device.